Abstract: Cytomegalovirus (CMV) is the most common opportunistic infection in solid organ transplant recipients, particularly in lung transplant recipients (LTRs). Active CMV infection is associated with acute and chronic rejection (bronchiolitis obliterans syndrome), with donor+/recipient- (D+R-) mismatched lung transplant recipients (LTRs) at highest risk for CMV disease and increased mortality. The mechanisms by which D+R- LTRs develop and maintain protective CMV-specific T cell immunity, particularly within the lung allograft, remain incompletely understood. Our preliminary data reveal a striking induction of the transcription factor T-bet, a central regulator of Type-1 immunity in mice, during human primary CMV infection. Our central hypothesis states that optimal protective CMV-specific effector memory (TEM) is T-bet-dependent/polyfunctional, and necessary for viral host defense in the lung and other tissues during acute and chronic infection. To test this hypothesis, in SA1 we will determine the role of T-bet in the regulation of TEM cell function and host defense during human and murine CMV infection. Because we unexpectedly detect CMV-specific CCR7+ TCM cells in human and murine lung airways during active infection, we will determine the relationship between TCM and T-bet+TEM cells, and the role of TCM cells in pulmonary host defense in SA2. Our preliminary data indicates the immunodominance of CMV-specific CD8+ T cell memory changes over time. In SA3, we will determine whether differential CMV-specific T-bet+TEM cell responses predict acute primary/short-term versus long-term CMV protection in the absence of antiviral therapy. Our proposal is an extension and expansion of CMV-specific immune studies currently being conducted in D+R- LTRs by the Pl and his team under an active R21 award (R21 A1072537-O1A1). The PI, John McDyer, MD, is a K08 awardee, transplant pulmonologist, and immunologist, who is strongly committed to understanding CMV pathogenesis, viral immunity, and treatment of CMV infection in lung transplant recipients. He has assembled an expert team of collaborators/consultants in virology, biostatistics, flow cytometry/CMV immunity, and has established a murine CMV (MCMV) model of infection in his laboratory to complement human studies and further test mechanisms in MCMV host defense. This award will provide a foundation for novel translational work in a unique human CMV infection model, and along with MCMV model studies, address clinically relevant issues in viral host defense. Improved knowledge and analysis of CMV-specific immunity in high-risk LTRs may enhance our clinical ability to risk-stratify these challenging patients, and potentially impact future antiviral therapy practices.